This application claims priority to German Application 100 07 994.6, filed Feb. 22, 2000, the teachings of which are incorporated herein by reference.
Prior art injection methods have used internal gas pressure to produce molded thermoplastic parts. Such a method is known, for example, from U.S. Pat. No. 4,101,617, issued to Friederich on Jul. 18, 1978. A pressurized fluid is here introduced into the still melted plastic. The pressure generated in the interior of the melt presses the melt against the cavity wall of the injection molding tool; sink marks due to volume contraction during cooling are thus avoided.
Nitrogen usually is used as the pressurized fluid which is injected into the melt. It has the advantage that it is an inert gas and thus does not react chemically with the hot melt. However, this entails the disadvantage that nitrogen generally is quite expensive to produce. Either the injection molding apparatus has to be supplied with nitrogen from bottles or else the gas has to be obtained on-site by means of molecular filters, if larger quantities are needed.
The economic requirements for the process are becoming more and more stringent, calling for ever shorter injection molding cycles together with the greatest possible improvement in the quality of the resulting molded parts. Various proposals have been made to achieve shorter cycle times:
European Patent EP 0 400 308, issued on Jun. 23, 1993, proposes that after the gas has been injected into the melt, it is again allowed to exit at a point removed from the injection point; then the gas is caused to circulate through the resulting cavity. A cooler is integrated into the gas circulation. This should cool the melt faster, since cooled gas is introduced in a closed circulation.
On the other hand, the German Patent DE 42 19 915, issued on Dec. 23, 1992, is based on the idea of injecting cooled gas into the melt. It specifically specifies that the gas be cooled to temperatures down to xe2x88x92160xc2x0 C. In this way, the plastic material is supposed to cool as fast as possible; the time interval between injecting the melt into the mold until the product is removed from the mold is thus reduced.
The previously known methods have the disadvantage that, on the one hand, expensive nitrogen is used now as before and, on the other hand, despite all these measures, the cooling effect of the gas is limited due to its limited heat capacity. Independently of this, but due to the great viscosity difference between the melt and the gas, the problem arises that sometimes flow marks appear on the surface of the molded part, so as to impair the quality of the finished part.
It is therefore desirable to further develop the injection molding method of the generic type, in such a way that the above disadvantages are minimized or avoided. The method and the associated apparatus should make it possible to avoid using expensive nitrogen. Furthermore, it should accomplish the shortest possible cooling time, which significantly shortens the injection molding process. Finally, the method also should assure that the melt flows into the injection molding tool as homogeneously as possible, so that change-over marks are avoided as much as possible.
In terms of method, the invention in one embodiment includes a method for injection molding thermoplastic molded parts, having at least one cavity. The method includes: a) injecting thermoplastic melt from an injection unit along a melt flow path into a cavity of an injection molding tool; b) injecting a fluid into the still liquid plastic material, so that the plastic material is pressed against walls of the cavity; c) allowing the plastic material to cool until it forms a self-supporting molded part; and d) removing the molded part from the cavity of the injection molding tool. The cavity is filled with plastic melt, according to step a), in such a way that the cavity is not filled completely with plastic melt; and the fluid, which is injected into the still liquid plastic material in accordance with step b) is a liquid with a high heat capacity wherein the surface of the cavity is not covered completely until the fluid is injected into the melted plastic material.
The central idea of the invention therefore is to inject into the melt a fluid with a high heat capacity. In combination with filling the cavity partially, plastic material first of all is used more efficiently. Filling the cavity partially with melt in combination with using a fluid with a high heat capacity achieves the result of speeding up the cooling process, so that the cycle time of the injection molding cycle can be significantly reduced. Surprisingly, it has here appeared that, due to the cited characteristics, hardly any flow marks appear, which otherwise must be feared and are observed with the internal gas pressure method. This is due to the fact that the viscosity of the liquid melt is comparable to that of the injected fluid with a high heat capacity.
According to a first development, it is specified that, while the process step b) of is executed, a portion of the still melted plastic material is displaced from the cavity into a demoldable secondary cavity. The flow of plastic material from the cavity into the secondary cavity is controlled by valve means, which are opened and closed according to a prescribed timing This very specifically influences the overflow of melt from the main cavity to the secondary cavity.
The fluid can be injected along the melt flow path, through a sprue region, into the cavity, or alternatively directly into the cavity by means of an injection nozzle.
In the second case, it can be advantageous that, while step b) is being executed, the injected fluid drives back a portion of the plastic material situated in the cavity toward the injection unit and out of the cavity.
The flow behavior of the melt and the control of its behavior are further improved if, before the thermoplastic is injected according to step a) a pressure greater than the ambient pressure is built up in the cavity by introducing a gas. This gas pressure can be dissipated only gradually during the above step b). The dissipation of the gas pressure can be controlled or regulated depending on how the injection pressure of the liquid increases during the injection process.
As a process variant, it can also be proposed that, after the fluid has been injected and before or while the plastic is allowed to cool, more pressurized gas is introduced into the cavity formed by the fluid.
An advantageous design has proven to be such that the fluid is temperature-controlled before it is injected into the still melted plastic material. The idea here is that the fluid is cooled to a prescribed temperature range. An especially advantageous temperature range is between about 0xc2x0 C. and about 20xc2x0 C., preferably between about 4xc2x0 C. and about 15xc2x0 C. But it may also be necessary, especially in the case of materials which are damaged by shock-like cooling, to heat the fluid to a prescribed temperature range. Especially advantageous temperature ranges here are between about 20xc2x0 C. and about 150xc2x0 C., preferably between about 40xc2x0 C. and about 100xc2x0 C.
The inventive concept ascribes special importance to the removal of the injected fluidxe2x80x94if possible still in the injection molding apparatus. According to the invention, there are several possibilities here.
The first possibility is that, after the material has been allowed to cool and before it is removed from the mold, the following process step is performed: cxe2x80x2) introducing compressed gas, preferably compressed air, along the path over which the fluid was injected into the plastic material, and blowing out the fluid from the cavity of the molded part at least at one blowout point, which is situated at a point which is remote from the point where the fluid is introduced.
The blowout point preferably is disposed toward the end of the flow path of the plastic material.
Alternatively, the arrangement can be such that, at the moment in question, the following process step is performed: cxe2x80x3) introducing compressed gas, preferably compressed air, at a gas input point, which is remote from the point at which the fluid was injected into the plastic material, and blowing out the fluid from the cavity of the molded part through the point at which the fluid was introduced into the plastic material.
Besides blowing out the fluid from the cavity, suctioning out the fluid is also possible. This process is such that the following process step is performed: cxe2x80x2xe2x80x3) applying a vacuum to the point which has a fluidic connection with the fluid-filled cavity, so as to suction the fluid situated in the cavity out therefrom.
It is here of advantage to apply the vacuum to the injection nozzle for the fluid.
Appropriately controlling or regulating the media pressures can beneficially affect the molding process while the melt and the fluid are being injected and also while the material is solidifying. It is therefore advantageously specified that, during steps b) and/or c) pressure is exerted on the fluid that has been introduced into the plastic melt, this pressure being regulated or controlled in accordance with a specified time profile. The pressure profile can here be controlled or regulated according to a pulsating rising and falling pattern; such as a sinusoidal pressure pattern, coursing about a given constant pressure. Alternatively, it can also be specified that the pressure is regulated or controlled during a first time phase at a first, preferably constant level and, during a later, second time phase, at a second, preferably constant level, such that the first pressure level is lower than the second one. Water or oil can be used as fluids with a high heat capacity.
The injection molding apparatus to perform the inventive method comprises an injection unit, an injection molding tool with a cavity, which has a melt flow path, over which the melted plastic material can be introduced into the cavity, and means for injecting a fluid.
The inventive apparatus includes means for removing, especially suctioning out, the fluid situated in the cavity of the molded part, the fluid being a liquid with a high heat capacity.
Preferably, the means includes a nozzle through which the fluid can be blown out from the cavity. Alternatively, the means is an apparatus for creating a vacuum, which can be connected to the means for injecting the liquid.
Technical advantages are obtained if the apparatus contains an injection element by means of which fluid or gas can optionally be introduced into the cavity of the injection molding tool. The injection element therefore can be switched from one medium to another.